1. Field of the Invention
The subject invention relates to a piston for a diesel engine and method of making such a piston having a crown specially treated to resist thermal oxidation degradation and, more particularly, to such a piston of the steel type used in fuel-injected diesel engine applications.
2. Related Art
A diesel engine is a reciprocating-piston engine operating on the well-known thermodynamic cycle in which air is compressed, fuel is injected into the compressed charge, the auto-combusting mixture expanded to do work on the piston, and the products exhausted at completion of the cycle. In large steel pistons such as used in diesel truck applications, it is common to utilize a multiple-orifice nozzle to inject fuel during the combustion process. The nozzle with multiple orifices is located as centrally as possible above the piston crown and discharges fuel in a radial spray pattern. A depressed bowl in the piston crown is designed to ensure that the air-fuel mixture formed from the injection spray and the rotating air during injection completely fills the combustion space for optimal performance. If the air-fuel mixture fails to completely fill the bowl in the piston crown, both air utilization and power output will suffer. As a result, there will be a substantial decrease in the anti-polluting emission characteristics. Likewise, if there is an overlap and the mixture extends beyond the space between the individual injection events, the resulting excessive local fuel concentration will lead to air deficiencies and increased soot formation, again, decreasing the anti-polluting emission characteristics of the engine.
In addition to these timing issues, another problem contributes to a loss of the anti-polluting emission characteristics designed for the piston. Because the fuel injected into a diesel engine ignites spontaneously, high Cetane Number fuels are required. The burning fuel plumes generate intense heat. The bowl formed in the crown of the steel piston typically experiences oxidation in areas in close vicinity to and/or on the top edge of the combustion bowl, i.e., the lip-like interface between the bowl and the flat top rim of the piston crown. The result is a plume of radially extending torches extending from the multiple-orifice nozzle. This torching effect oxidizes the steel up to Fe2O3 status, and the resultant oxides have no adherence to the underlying, unaffected steel substrate. Mechanical expansion/contraction processes eventually dislodge this oxidized layer in a “flaking” manner. Over time, an eroded area can be seen with the naked eye. This change in the shape of the bowl lip causes disturbances in the combustion process and contributes to a loss of the anti-polluting emission characteristics designed into the combustion bowl of the piston crown. Besides, the eroded areas weaken the piston structurally. Piston flexing, expansion, and contraction may induce radial cracks which propagate and could eventually lead to piston functional failure.
Various attempts have been proposed to address the issue of bowl lip oxidation resulting from the intense heat release by combusting fuel. For example, some have proposed to fabricate the entire piston crown from a specially formulated alloy designed to combat oxidation and corrosion. However, the piston crown in a large diameter piston for truck applications requires a significant amount of material. Such specially formulated alloys would significantly increase the cost of a diesel piston.
Other prior art attempts to address this issue include U.S. Pat. No. 5,958,332 to Hoeg, granted Sep. 28, 1998. In this example, a special plate fabricated from a high temperature, corrosion resistant alloy is welded to the critical areas of a piston or other engine component. However, the loose-piece fabrication of a special alloy plate significantly increases the cost of the piston assembly, as well as adding numerous handling and assembly steps to the fabrication process. In examples, proposals have been made to shrink-fit an annulus of high temperature resistant steel or even a ceramic-based material into the combustion bowl. However, the same restrictions outlined before apply. Accordingly, there is a long-felt and as yet unsolved need to address the issue of piston crown degradation in low-cost steel pistons resulting from oxidation and the intense heat released by diesel engine combustion in close proximity to the combustion bowl lip. A commercially practical solution must be convenient to implement without increasing the overall product or manufacturing costs, while retaining long term piston emission compliance performance.